CA1179382A

CA1179382A - Ceramic foam filter and aqueous slurry for making same

Info

Publication number: CA1179382A
Application number: CA000394743A
Authority: CA
Inventors: Jerry W. Brockmeyer
Original assignee: Schweizerische Aluminium AG
Current assignee: Alcan Holdings Switzerland AG
Priority date: 1981-01-22
Filing date: 1982-01-22
Publication date: 1984-12-11
Also published as: BR8200296A; EP0058812A2; NO169602B; ZA819001B; US4343704A; EP0058812B1; JPS57140613A; JPH0147207B2; DE3172090D1; NO169602C; NO820160L; ATE15149T1; EP0058812A3

Abstract

ABSTRACT OF THE DISCLOSURE
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The present invention resides in an improved ceramic foam filter particularly useful for filtering molten metal and also an improved aqueous slurry for use in preparing same.

Description

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B~KGROUND OF ~HE INVENTION
Molten aluminum in practice generally contains entrained solids which are deleterious to the final cast metal product. ~ese entrai~ed solids usually derive from three sources. Some are particles of aluminum oxide which are drawn into the liquid stream frcm the float-ing oxide layer on its surface, and some entrained particles are frag-ments of furnace lining, transfer trough and other portions of the molt-en aluminum handling equipment which are eroded and entrained in the flowing aluminum streaml and scme particles are precipitates of insol-uble impurities such as intermetallics, ~orides, carbides or precip-itates of other alumin~lm com~ounds, such as chlorides. When these incl-usions apFe~ar in the final cast product af-ter the molten aluminum is solidified, they cause such final product to be less ductile or to have poor finishing characteristics. Accordingly, it is desirable to remove ; 15 entrained solids from the molten aluminum stream beore it is cast into a solid body which may be used as such or sub~ected to forming operations such as rolling, forging, extrusion, etc.
Filtering processes to remcve entrained solids frcm liquids are acccmplished by passing the solid-laden liquid through a porous filte~ medium that will not pass the solids. Filte~in~ molten metal in general, and molten aluminum in particular, creates special problems because the liquid is so aggressive that it is difficult to find a filter medium capable of withstanding it.
In general, two methcds of filtering are used for removing entrained solids frcm molten aluminum alloys be~ore casting. The most common filter medium is an open weave glass cloth screen placed in the metal transfer trough, around -the SpClt or even in the molten metal pool in the top of the solidifying ingot. m ese clvth screens are able to remove only the larger sizes of inclusions from the metal and are easil~ ruptured during use because the glass fibers become very weak at the temperature of molten aluminum. In another prior art procedure, molten aluminum is filtere~ tnrough a bed of loose alumuna particles, for example, of tabular alumina, but it often suffers from the drawbacks normally associated with bed filters in that it passes too many solids, there is a strong tendency to channeling whicn prevents efficient use, and pore size of the filter is not easily c~ntrolled but rather readily changes under conditions of use so that, even when originally of proper dimension, it cannot be efficiently maintained. In addition, the metal mLst be kept constantly molten when the filter is not in use.
An impro~ed method for filtering and removing entrained solids from molten aluminum alloys is disclosed in U.S. Patent 3,947,363, assigned to the assigned to the assignee of the instant invention, and employs a ceramic foam filter having an open cell structure characterized by a plurality of interconnected vo:ids surrounded by a web of ceramic. l'he camposition of the ceramic foam material employed is an alu~ina-chromia compositioll comprising preferab~y 45 to 55~ A12O3; 10 to 17% Cr203; 0.5 to 2% bentonite and from 12 to 17% of a ceramic binder such as aluminum orthophosphate. The use of the ceramic foam filter disclosed in the '363 patent greatly increased filtration efficiency however the use of a ~ larger percentage of chromia in the ceramic foam composition results in `I a number of disadvantages. One disadvantage is that chromia is relatively high in cost compared to alumina and is only available from limited areas and therefore a potential supply problem exists. A further problem is an effluent problem since Cr+6 is a known carcinogen and while the chramia does not contain measurable Cr~6 it does remain a concern. In addition to the foregoing, when large size filters are produced in the preferred thickness of two inches ~2") there is a problem with the strength of the filter. The use of thicker filters would add to the cross sectional strength but would likewise result in a substantial cost increase without any noticeable benefit on filtration. Naturally, in ligh-t of the foregoing, it would be highly desirable to increase the intrinsic strength of the ceramic co~Fosition while at the same time eliminating chramia from the cQmposition.
Accordin~ly, it is a principal object of the present invention 1:~L'7~
to provide a ceramic fo~m filter which has considerable strength so that it can be used as a filter for a variet~ of molten metals.
I-t is an additional objec-t of the present inven~ion to provide a cerarnic foam filter characterized by strength integrity which can be prepared at reasonable cost.
It is a further object of the present invention to provide a ceramic foam filter as aforesaid which does not contaminate the melt.
It is a still further object of the present invention to provide a ceramic foam filter as aforesaid which obtains high filtration efficiency.
Further objects and advantages of the present invention will appear hereinafter.
, UM~RY F ~ INVENTION
In accordance with the present invention it has now been found that the foregoing objects and advantages may be readily obtained.
The present invention provides a highly efficient, ceramic foam material for use in filtering rnolten metal, especially molten aluminum.
e ceramic foam material of the present invention is characterized by having an open cell st~ucture with a plurality of interconnected voids surrounded by a web of said ceramic material. The cornposition of -the ceramic foam material of the present invention is as follows: 55 to 70% A1203;

2 to 10~ rnicron-sized reactive alumina; 1 to 5% montmorillonite; 1 to 10%
ceramic fibers; and preferably contains fro-m 2.5 to 25% of a ceramic binder or air setting agent which is substantially nonreactive to the rrolten metal.
In addition to the foregoing, -the present invention contemplates an aqueous slurry for use in ~reparing the foregoing high temperature resistant cerarnic foam, wherein said slurry contains the materials listëd above in the foregu mg amounts.

DETAILED DESCRIPTION
In accordance with the present invention the ceramic foam filter 9;~
described above has been found to be particularly useful in filtering molten metal, especially molten aluminum. The high tem~erature resistant charclcteristics of the filter of the present invention are particularly useful ~mder the severe use conditions encountered i~ the filtration of molten metaI. Furthermorel the composition of the ceramic filter of the present invention has been found to be such that there is no difficulty wi-th contamination of the me-tal.
The ceramic filter of the present invention is a low cost material which may be conveniently used on a disposable basis. The molten metal is poured through the ceramic foam material at a rate of from 5 to 500 and preferably 30 to lO0 cubic inches per square inch of fil-ter area per minute and entrained solids are thereby removed from the molten - metc~l. The filter is prepared in accordance with the general procedure outlined in U.S. Patent 3,893,917, which is assigned to the assignee of the present invention. As described therein, an open cell, flexible organic foam material is provided having a plurality of interconnected voids sur-rounded by a web of said foam material. me aqueous slurry described above is prepared and the foam material is impregnated therewith so that the web is coated therewith and the voids are substantially filled therewith.
The impregnated rnaterial is then compressed to expel about 80% of the slurry and the balance is uniformly dis-tributed throughout the foam material, preferably so that some pores are blocked in a uniformly distributed manner to increase the tortuosity. The ccn~pression is released so that the web remains coated with the slurry and the material is dried. The dried matePial is then heated to first burn out the flex~ble organic foam and then sinter the ceramic coating, thereby providing a fused cer~nic foam having a plurality of interconnected voids surrounded by a web of bonded or fused ceramic in the configuration of the flexible foam. The foregoing procedure is fully described in the aforesaid U.S. Patent.
The princi~al c~nponent of the cer2mic foam rnaterial of the present invention is A1203. In accordance with the present i7~ention the A1203 is present in two forms, the principal form bein~ calcined Al~03 in a size range frcrn about 100 mesh to 500 mesh, preferably about 325 mesh average in amounts of frcm 55 to 70%, preferably 60 to 65%. A1203 is particularly desirable for use as a ceram~c foam filter since it is not attacked by molten aluminum or molten copper; whereas silicon is attacked by these materials. Furthermore, the alumina has reasonable strength to stand up to chenLical attack and structural and/or mechanical strength to stand up to particular environmental conditions. me A1203 is also present in the form of micron-sized reactive alumina in amounts of frcrn about 2 to 10%, preferably 2 to 5%. The use of micron-sized reactive alumina in the aTx~unts set forth above is critical in the present inv-ention in that the reactive alumina aids in achieving a ha~geneously distributed bonding phase arld a suitable slurry rheology while allowing the plastic ma-terials incorporated in the slurry, which tend to be reactive to the molten metal, to be held to a minimum.
Mont:rnorillonite in An Amount of frcm about 1 to 5%, preferably 1 to 3% has been found to be a particularly important additive to the slurry carnposition of the present invention. The montrnorillonite is a highly plastic material wnose caTposition is approximately A1203.4SiO2.H20.
It has been found that the montrnorillonite is more plastic than bentonite.
me use of a srnall percentage of hignly plastic montrnorillonite rather than bentonite all~s for a suitable thixotropic rheology to be achieved while actually increasing the intrinsic strength of the resulting filter ccrnposition. I~ montrnorillonite acts in synergism with the micron-sized reactive alumina to achieve a homogeneausly distributed bonding phase while at the same time improving the intrinsic strength of the resulting filter element.
In addition to the foregoing, it is an additional critical feature of the present invention that a ceramic fiber material be added to the aqueous slurry in a range of frcm about 1 to 10%, preferably 1 to

3%. it has been faund that an addition of more than 10% }~y ~eight of ceramic fibers to the slurry results in a clumping of t~ fibers and therefore good dispersion of the fi~ers in the slurry cannot be ob-tained.

~7~8'~
An addition of ceramic fibers in excess of 3% shows a relatively minor increase in strength. In the preferred 1 to 3% range significant strength improvement is obtained and the fiber is easily dispersed throughou-t the slurry as no clumping of the fibers occurs. ~he ceramic fibers act as a crack grc~th inhibitor and thereby actually increases the intrinsic strength of the resulting filter element. The preferred ceramic fiber for use in the slurry of the present invention is alumina-silicate fibers due to their generally low cost. ~c)wever, additional refractory fibers such as alumina and zirconia as well as others cauld readily be used.
In addition to the fc~egoing, one provides frcm 2.5 to 25%~
of an air setting agent which is substantially nonreac-tive to the molten metal. The air setting or bonding ag-nt sets up or hardens the ceramic slurry without ~le need for heating, and preferably by drying , normally by a chemical reaction, while heating to moderate temperatures. The preferred air setting agent is aluminum orthophosphate, preferably in the form of a 50~ aqueous solution. Other air setting agents which may be employed include, for example, magnesium orthoborate, al~ num hydroxy chloride, etc. Alkaline metal silicates such as sodium silicates may be employed at least in part; ncwever, these æ e less desirable since melting and conse~uent loss of set occurs at tempera~lres arcun~ 1500F.
Furthermore, the silicon contents thereof, and perhaps the sodium content, may becane dissolved in the melt. Similæ ly, ethyl silicate and other phosphates may be employed but are~less desirable. Aluminum Qrthophosphate is particularly preferred due to its very desirable combination of proper-ties, that is, nonreactivity, stability over a wide range of temperatures and good setting properties.
As indicated hereinabove, the air setting agent is preferably added as an aqueous suspension including equal parts of binder and water particularly in the case of aluminum orthophosphate. The binder provides strength before the formation of the ceramic kond, that is, after the burning off or volatilization of the web of flexible foam material~ me binder material provides sufficient strength to hold the mixture to~ether for formation of the final product. In fact, the stability and strength of the chemical bond provided by the preferred air setting agent is sufficient for many applications to enable use of the product at this stage without high temperature sintering. This strength is substantial and exists over a wide temperature range. m e preferred e~bodiment util-izes fran 15 to 25% of aluminum orthophosphate.
- As indicated hereinabove, the slurry is an aqueous slurrywhich contains a certain amount of water in order to aid in controlling viscosity so that one can impregnate the foam material with the slurry and also to enable one to work conveniently with -the slurry. Generally from 10 to 40~ water is present in the slurry with same of this water being added as an aqueous solution of the aluminum orthophosphate.
The resultant product is a bonded ceramic foam material having an op~n cell structure characterized by a plurality of inter-connected voids ~surrounded by a web of ceramic. Naturally, the ceramic foam may have any desired configuration based on the configuratlon needed for the particular molten metal filtration process. Although naturally these configurations can be many and varied, particular con-figurations may be preferred for filtration in a transfer trough between the furnace and the casting mold in filtering molten alu~unum. A wide variety of suitable configurations may be readily an~ conveniently prepared in view of the flexibility afforded by the preparation process utilized herein. It is a particular advantage of the ceramic foam material of the present invention that said ceramic foam has substantial high temperature strength and is capable of withstanding attack by molten metal. Furthermore, the filter of the present invention is advantageous in that excessive heads of molten metal are not required in order to start the filtration process.
In accordance with the present invention the specific features thereof will be more readily understandable from a oonsideration of the follcwing illustrative example.

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E~PLE
A polyurethane foc~n material was provided havirlg a thickness of 2 inches and containing 30 pores per linear inch. An aqueous ceramic slurry was provided having the follcwing CQn~ositiOIl. 62.3% of 325 mesh calcined A1203, 3.6% rnicron-sized reactive alumina, 1.1% montmorillonite, 1.0% ceramic grade alununa-silicate fiber/ 22.8% alun~inum orthophosphate as an aqueous solution with an equal an~unt of water and an additional 9.2% water. The foam rnaterial was i~nersed in the slurry and kneaded to rernove air and substantially fill the voids with the slurry arld also to 10 coat the fibrous webs of the foam with the slurry. The resul-tant impreg-nated foam was subjected to cQmpression to squeeze appro~imate:Ly 80n6 of the slurry out of the foam by passing the impregnated foam through preset rollers. The mcaterial was then dried and fired to provide an open cell ceranùc foam material having a configuxation of the origunal polyurethane 15 foam material. The resulting filter elernent was then campared to a filter element prepared from an aqueous slurry comprising 47% A1203, 136 Cr203, 3.5% koLine, 1.0% bentonite and 14.5% aL~ninum orthophosphate added as an aqueous solution with an equal an~unt of water as set forth in EXample I
i of previously noted U.S. Patent 3,947,363. ~ne canpressive strength of the filter element of the present invention was found to be 50% greater than tilat filter element prepared fxom the alununa-chromia composition and bending strengths were found to be 10 to 20% higher. In addition, the filter el~nent of the present invention was used successfully in filtering oth molten aluminum and molten copper with results comFaxable to the c~lumina-chrornia filter element.
; This invention rnay be embodied in other foLms or carried out in other ways without departing frcm the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all char~es which cane within the n~3~nin~ and range of equivalency are intended to be e~braced therein.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A ceramic foam filter for use in filtering molten metal having an open cell structure characterized by a plurality of interconnected voids surrounded by a web of ceramic, said foam having the following composition:
55 to 70% A1203, 2 to 10% micron-sized reactive alumina, 1 to 5% mont-morillonite and 1 -to 10% ceramic fibers.

2. A filter according to claim 1 containing from 2.5 to 25% of an air setting agent which is substantially nonreactive to the molten metal.

3. A filter according to claim 1 containing from 60 to 65% A1203.

4. A filter according to claim l containing from 2 to 5% micron-sized reactive alumina.

5. A filter according to claim l containing from l to 3% ceramic fibers.

6. A filter according to claim 5 wherein said ceramic fibers are alumina-silicate fibers.

7. A filter according to claim 2 wherein said air setting agent is aluminum orthophosphate.

8. A filter according to claim 7 containing from 15 to 25%
aluminum orthophosphate.

9. An aqueous slurry for use in preparing a ceramic foam having the following composition: 55 to 70% A1203, 2 to 10% micron-sized reactive alumina, 1 to 5% montmorillonite and 1 to 10% ceramic fibers.

10. A slurry according to claim 9 containing from 2.5 to 25% of an air setting agent which is substantially nonreactive to the molten metal.

11. A slurry according to claim 9 containing from 60 to 65% A1203.

12. A slurry according to claim 9 containing from 2 to 5% micron-sized reactive alumina.

13. A slurry according to claim 9 containing from 1 to 3% ceramic fibers.

14. A slurry according to claim 13 wherein said ceramic fibers are alumina-silicate fibers.

15. A slurry according to claim 10 wherein said air setting agent is aluminum orthophosphate.

16. A slurry according to claim 15 containing from 15 to 25%
aluminum orthophosphate.